Low molecular weight polycarboxylic acid polymers and their salts are useful as dispersants, scale inhibitors, detergent additives, sequestrants, etc. Generally a molecular weight below 50,000 is necessary for effective performance and often very low molecular weights below 10,000 are most effective. It is common to use chain transfer agents in the polymerization reaction to produce low, and especially the very low, molecular weight polymers. Hypophosphorus acid or its salts (commonly sodium hypophosphite) are a particularly desirable chain transfer agent, chosen primarily because they introduce phosphinate and phosphonate functionality into water soluble polymer molecules which confers superior performance properties in some applications. As used hereinafter and in the appended claims the term "hypophosphorous acid" is intended to include salts thereof, unless a clearly different meaning is indicated. The use of hypophosphorous acid and its benefits have been taught in several U.S. Pat. Nos. 2,957,931, 4,046,707, 4,105,551, 4,127,483, 4,159,946 and 4,621,127.
Inefficiency is a problem that is common to all the prior use of hypophosphorous acid as a chain transfer agent. Much of the agent does not specifically become part of the transfer process. A significant portion is not incorporated into the polymer and remains unreacted or is converted to other inorganic species like phosphorus acid or its salts. As a consequence, high levels of hypophosphorous acid are required to obtain low molecular weight. Since hypophosphorous acid is a relatively expensive material, it becomes a costly route to low molecular weight polymers. In the case of preparing very low molecular weight polymers, the levels of hypophosphorous acid required may be prohibitively expensive.
A second disadvantage arising from the inefficiency is the significant amounts of unreacted hypophosphite or other inorganic residues present in the reaction product. These salts do not contribute to performance thereby diluting the activity of the reaction product. In some cases, such as preparing concentrated clay slurries, these salts can interfere with the dispersing process.
Another disadvantage of prior usage of hypophosphorous acid resides in the mix of polymer species formed. The reaction products include dialkyl phosphinate polymers and monoalkyl phosphinate and phosphonate polymers. In the prior art processes, only about 50% or less of the hypophosphorous acid is incorporated into the dialkyl species. Since the dialkyl species are believed to have greater biodegradability than the monoalkyls of equal molecular weight, the prior processes give products with relatively poor biodegradability.